This specification generally relates to apparatus and methods for analyzing properties of materials, and more specifically, for performing spectroscopic measurements.
Detection and analysis of microscopic particles by spectroscopic determination of their chemical constituents or composition can be used in various applications, such as detection of a minute amount of a substance, especially in small areas less than 10 xcexcm. Such techniques may be used in, for example, detecting narcotic or explosive particles on a surface, presence of biological substances in a sample, examining the process uniformity of diffusion during fabrication of semiconductor substrates or crystal wafers, or measuring properties of samples under extreme conditions such as high temperature, high pressure, or both.
One widely-used spectroscopic technique uses a Fourier-transform infrared (FTIR) spectrometer which is based on a Michelson interferometer. A beam from a light source is split into two beams. One is reflected by a fixed mirror and another one is reflected by a movable mirror. The two reflected beams are recombined by the beam splitter as a single beam whose intensity is modulated due to interference. The interference pattern is a function of the relative spatial displacement of the movable mirror relative to the fixed mirror and is associated with the Fourier transform of the spectrum of the recombined beam.
When this recombined beam transmits through or reflects from a sample, it carries the spectral information of the sample through optical interaction with the sample by spectral components therein. Hence, spectral information about the sample can be extracted by performing a Fourier transform of the intensity variation of the received recombined beam.
One of important components of the FTIR spectrometer is the light source. The radiation spectral range of the light source ultimately determines the spectral range of the spectrometer. A wide-band light source, such as a heated ceramic material, is often used. This wide spectral range of the light source allows parallel and simultaneous processing of spectral information at all wavelengths by the FTIR spectrometer. As a result, FTIR spectrometers can provide high-speed spectral analysis and have been used in various on-line process monitoring applications. However, certain disadvantages are also associated with such a wide-band light source.
For example, the light intensity from a wide-band light source at each wavelength within the spectral band is quite low. This limits the detection sensitivity in measuring minute quantities of substances in a sample since the signal at a particular wavelength of interest can be so weak as to produce an unacceptably low signal-to-noise ratio. Use of a long integration time can be impractical in many applications.
In addition, the spatial resolution of the spectroscopic analysis in such FTIR spectrometers is also limited when analysis of microscopic samples with small dimensions (e.g., less than 10 xcexcm) is desired. This type of microsampling application presents a number of technical obstacles to conventional FTIR spectrometers. For example, since the spatial extent of samples is small, only a limited amount of light of all wavelengths within the spectral band can be directed through or onto the samples. Also, the light intensity at each wavelength is further limited since it is only a small fraction of the limited total amount of light.
One way to obviate the above and other limitations associated with the conventional FTIR spectrometers is to replace the conventional wide-band light sources with the synchrotron radiation emitted by accelerated charged particles. See, for example, Lu et al, Trans. Amer. Geophys. Union. 77(46), F661 (1996). However, a synchrotron source is essentially a circular particle accelerator and is prohibitively expensive and physically large. Although an FTIR spectrometer based on a synchrotron source may have limited use in large-scale scientific research, use of a synchrotron source is impractical for most scientific and industrial applications.
A further limitation of a FTIR spectrometer is the optical condensing and collecting units associated with the use of a wide-band light source. Special care is required to design such optical units in order to reduce various optical aberrations, including chromatic aberration. Schmidt-Cassegrain systems, which have two concentric spherical reflectors, are often used for both focusing the wide-band light onto the sample and collecting the light from the sample. This can significantly complicate the design and maintenance of the optical system in FTIR microsampling spectrometers.
In recognition of the above, the present disclosure provides apparatus and method based on a different approach to detection and analysis of microscopic samples in minute quantities. The apparatus includes a special combination of a tunable optical parametric oscillator (OPO) laser and an absorption optical system to form a tunable absorption spectrometer.
One embodiment of the apparatus includes a beam splitter to divide a monochromatic beam from the OPO laser into a reference beam and a sampling beam, a reference photodetector to convert said reference beam into a reference electrical signal indicating an intensity of the sampling beam, a sampling aperture with a dimension less than 10 xcexcm positioned to limit a spatial extent of the sampling beam, a sample holder adapted to hold a sample relative to the sampling aperture to expose the sample to the sampling beam, and a sampling photodetector to convert transmitted light from the sample into a sampling electrical signal. A control circuit is also provided to control the wavelength of the OPO laser and to process said reference and sampling electrical signals to produce absorption data as a function of the wavelength of the sampling beam.
One embodiment of the microsampling method includes the following steps:
generating a tunable monochromatic laser beam from optical parametric oscillation;
reducing the dimension of the laser beam to increase an intensity above a specified level;
dividing the laser beam into a reference beam and a sampling beam;
measuring and using the intensity of the reference beam to indicate an intensity of the sampling beam;
using a sampling aperture with a dimension less than 10 xcexcm to limit spatial extent of the sampling beam;
exposing a sample to the sampling beam that transmits through the sampling aperture;
measuring the intensity of transmitted light from the sample; and
tuning the wavelength of the laser beam to obtain the absorption spectrum of the sample.
These and other aspects and associated advantages will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.